1. Field of the Invention
The present invention relates to a semiconductor device which is intended for use in transistor modules and the like. More particularly, the present invention relates to a semiconductor device constructed to prevent the leakage of filler from the device, and the exposure of the filler to water.
2. Discussion of the Related Art
A conventional semiconductor device has been embodied as a package formed by combining a base, which functions as a radiator panel, with a case or housing made of an insulative material. In addition to the package (i.e., the base and the housing), the semiconductor device includes a semiconductor element chip, an external lead terminal, a gelled resin filler (e.g., a silicone resin), and a sealing resin (e.g., an epoxy resin). The semiconductor device is constructed by filling the housing with the gelled resin filler, such that the semiconductor element is embedded in a lower area inside the housing, applying the sealing resin on the gelled resin filler, and hardening the sealing resin.
The silicone gel or resin used as the gelled resin filler within the housing has a larger coefficient of thermal expansion than other sealing resin materials (such as the epoxy resin). Further, in the conventional semiconductor device, when the gelled resin filler expands in volume along with a rise in temperature resulting from a heat cycle of the semiconductor device, the excess volume of the gelled resin filler resulting from the expansion causes the generation of an internal pressure that cannot be relieved. As a result, stress induced by the internal pressure caused by the excess volume of the gelled resin filler may be applied to the semiconductor element chip, thereby causing the formation of cracks in the semiconductor element chip.
The same stress may also cause the gelled resin filler to break a bonded portion of the base and the housing, resulting in leakage of the filler from the semiconductor device. In particular, if the gelled resin filler suddenly expands due to an abnormal heat generation (such as the heat generation caused by an overcurrent flowing through the semiconductor element chip), the housing may break abruptly, thereby allowing the contents of the housing to splash out and affect ambient objects outside of the semiconductor device.
In view of the foregoing problems, a semiconductor device has been proposed wherein the housing of the semiconductor device is provided with a vent hole that penetrates to the gelled resin filler through the thermosetting resin for communication between the gelled resin filler and the outside air. This type of device is discussed in Japanese Utility Model Application No. Hei. 1-119534 as preventing breakage of the housing and leakage of the gelled resin filler from the semiconductor device. In accordance with the construction of this type of semiconductor device, the expansion in volume of the gelled resin filler that occurs along with the rise in temperature is absorbed by the capacity of the vent hole. At the same time, excessive pressure in the housing can be released to the outside air through the vent hole.
The provision of the vent hole in the housing reduces leakage of the gelled resin filler from the housing and prevents breakage of the housing in situations where the internal pressure of the housing increases. This result is obtained because the vent hole allows the absorption of increases in the volume of the gelled resin filler that occur upon expansion of the gelled resin filler. Nevertheless, the provisions of semiconductor device with a vent hole in the housing does not produce entirely satisfactory results. More particularly, the gel used in the semiconductor device having a vent hole has a high water absorptivity and, therefore, the gelled resin filler used in the semiconductor device may absorb moisture from the outside air through the vent hole and swell during use of the semiconductor device so that the dielectric strength characteristics of the semiconductor device are adversely affected.